How to Join two Armatures without losing constraints in Blender 2.8?
All constraints look for an armature and a bone. If you merged the &quot;B&quot; armature into the &quot;A&quot; armature, probably some constraint are looking for an &quot;X&quot; bone which was in the &quot;B&quot; armature, but now it is into the &quot;A&quot; armature. If all bones names are different, it should be enough to change all constraints armature target from &quot;B&quot; to &quot;A&quot;.